Gas separation apparatus and gas separation method

ABSTRACT

Using a distillation separator, a gas to be treated is separated into a plurality of gas groups having different boiling points. Then, the specific gases included in each of the plurality of gas groups separated at the first separating means and having similar oiling points are separated using a chromatographic separator. In this manner, specific gases can be separated from the gas to be treated containing a plurality of specific gases.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a gas separation apparatus andgas separation method for separating specific gases from a gas to betreated which contains a plurality of specific gases.

[0003] 2. Description of the Related Art

[0004] Conventionally, various gases are used in a semiconductormanufacturing processes depending on the particular process adopted. Forexample, perfluoro compound (PFC) gas which is a mixture containingfluorine compounds such as CF₄, NF₃, C₂F₆, C₃F₈, SF₆, and CHF₃ is usedas a reaction gas at the dry etching process or at the cleaning processfor a thin film forming device. In these processes, a discharge gas isproduced which contains the PFC gas.

[0005] Because these discharge gases such as PFC promote global warmingand thus cannot be discharged out of the line without furtherprocessing, various methods are employed for treating these gases. Thetreating methods include (i) decomposition in which the PFC gas isdecomposed by combustion, catalyst heating, or plasma decomposition;(ii) membrane separation in which these materials are separated by amembrane; (iii) distillation separation by subzero cooling in which theseparation is achieved taking advantage of the difference in the boilingpoints of the gases; and (iv) chromatographic separation in whichseparation is achieved by taking advantage of the difference in time forpassing through a chromatographic column.

[0006] However, in the decomposition method (i), there are shortcomingsin that a complete decomposition is difficult and the gas cannot berecovered for reuse because the gas is decomposed and discharged. In themembrane separation (ii), although nitrogen in the discharge gas can beremoved, the separation between PFC gases is difficult. In the subzerocooling separation method (iii), separation between gases which onlyhave a small difference in the boiling points, such, for example, as CF₄and NF₃ with the difference being only 1° C., is difficult. In aconventional PFC recovery device employing the subzero coolingseparation method, when recovering a mixture gas of CF₄ and NF₃, NF₃must first be decomposed and the remaining CF₄ alone is then recovered.However, the trouble with this method is that NF₃ which is the mostexpensive gas among the PFC gases cannot be recovered. In thechromatographic separation (iv), there is a problem that when three ormore PFC gases are present, these gases cannot simultaneously beseparated and recovered.

SUMMARY OF THE INVENTION

[0007] The present invention has been conceived to overcome theshortcomings of the conventional gas treatment methods mentioned above,and one object of the present invention is to provide a gas separationdevice and gas separation method capable of separating a mixture gashaving a plurality of constituents, in particular, three or moreconstituents, inexpensively and with high purity.

[0008] According to the present invention, a mixture gas to be treatedcontaining a plurality of gas constituents is first separated bydistillation separation into a plurality of gas constituent groups, eachof which group has similar boiling points, and then the specific gaseswithin each of separated gas constituent groups are separated bychromatographic separation. In this manner, a mixture gas having aplurality of constituents, in particular, three constituents or more,can be separated inexpensively and with high purity.

[0009] For example, when a mixture gas having a plurality of gasconstituents includes CF₄, NF₃, C₂F₆, and CHF₃, and nitrogen is anadditional gas constituent, the gases can be separated by distillationseparation into a first gas group which includes CF₄ (having a boilingpoint of −128° C.) and NF₃ (having a boiling point of −128.8° C.), asecond gas group which includes C₂F₆ (having a boiling point of −78° C.)and CHF₃ (having a boiling point of −82.2° C.), and a third gas groupwhich includes nitrogen (having a boiling point of −195° C.). Then, bychromatographic separation, the mixture gas of the first gas group canbe separated into CF₄ and NF₃. The mixture gas of the second gas groupcan similarly be separated by chromatographic separation into C₂F₆ andCHF₃. the separated gases of CF₄, NF₃, C₂F₆, and CHF₃ can respectivelybe recovered for reuse.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram showing the structure of a deviceaccording to the present invention.

[0011]FIG. 2 is a diagram showing the structure of a chromatographicseparator according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0012] In the present invention, the plurality of specific gases are,for example, PFC gases discharged from a semiconductor manufacturingprocess. The PFC gases include any one of a fluorine compounds having atleast one of the elements C, N, and S as the constituting element.Specifically, examples of PFC gases include CF₄, NF₃, C₂F₆, C₃F₈, SF₆,and CHF₃. The present invention is particularly effective when the PFCgases include at least the following three constituents: CF₄, NF₃, andC₂F₆ or CHF₃ or when the PFC gases include at least the following threeconstituents: C₂F₆, CHF₃, and CF₄ or NF₃. The present invention isespecially effective for separation of PFC gases containing both CF₄ andNF₃ which have similar boiling points or for separation of PFC gasescontaining both C₂F₆ and CHF₃ which have somewhat similar boilingpoints. The gas to be treated usually contains 0.1% to several percentPFC gas and nitrogen as the remainder gas.

[0013] The gas to be treated is first introduced to a distillationseparator 10 for separating the gases by distillation separation into aplurality of gas constituent groups each of which group has similarboiling points. As the distillation separator 10, any known device whichuses a distillation column can be used. That is, in the distillationcolumn, gas constituents included in the gas to be treaded are cooled torespective boiling points and liquefied to form a gas-liquid state sothat separation and purification can be performed. For example, amixture gas of two constituents having similar boiling points can begrouped as one group, and PFC gases can be separated and purified into aplurality of groups. More specifically, the gas to be treated isdistillation separated into three gas groups, that is, CF₄ and NF₃ eachof which has a boiling point near −128° C. constituting a first group,C₂F₆ and CHF₃ which have boiling points of −78° C. and −82.2° C.,respectively, constituting a second group, and nitrogen having a boilingpoint of −195° C. constituting a third group. The separated first gasgroup and second gas group are then each chromatographically separatedinto high purity specific gases by a chromatographic separators 12 a and12 b provided downstream of the distillation separator. Nitrogen in thethird gas group is recovered and reused. In the distillation separator10, it is possible to further obtain fourth, fifth, etc. gas groupsdepending on the composition of gas constituents in the mixture gas tobe treated.

[0014] The first and second gas groups separated at the distillationseparator 10 are then respectively introduced to a chromatographicseparators 12 a and 12 b for separating the specific gases constitutingeach gas group and having similar boiling points, by chromatographicseparation. That is, in the first group, a first specific gas (e.g. CF₄)and a second specific gas (e.g. NF₃) are separated and in the second gasgroup, a third specific gas (e.g. C₂F₆) and fourth specific gas (e.g.CHF₃) are separated. Because the chromatographic separation operation issimilar for both gas groups, the chromatographic separation operationwill be described below for separation of CF₄ and NF₃ in the first gasgroup.

[0015] As the chromatographic separators 12 a and 12 b, any knownchromatographic separator having a column filled with a given filler canbe used. The first gas group is passed through the separator 12 a. Inthis manner, this first gas group is separated into its constituentsbecause the constituents have different retention times due todifference in the affinity of the gas constituents with respect to thefiller. As a filler, for example, silica gel or molecular sieve can beused for separating CF₄ and NF₃. In the chromatographic separators 12 aand 12 b, nitrogen is used as a carrier gas and CF₄ and NF₃ areseparated by sequentially desorbing and discharging CF₄ these gasconstituents adsorbing onto the filler.

[0016] When a mixture gas fraction having both CF₄ and NF₃ is obtainedfrom the separator 12 a, it is preferable to return this fraction to theinlet side. For example, while passing the carrier gas, which is thenitrogen gas, a predetermined amount of mixture gas fraction can bemixed into the nitrogen gas, and a fraction containing CF₄ and afraction containing NF₃ can be separately collected.

[0017] It is also preferable to provide a plurality of columns in thechromatographic separator 12 a (or 12 b) to form a simulated-moving fedtype chromatographic separator in which the first gas group is suppliedto each column in sequence and each fraction is collected from eachcolumn in sequence. FIG. 2 shows a configuration example of asimulated-moving fed type chromatographic separator 1 in which fourcolumns 1 a, 1 b, 1 c, and 1 d are provided, and fractions are obtainedby supplying the first gas group to the columns in sequence. Forexample, nitrogen can be continuously supplied to the columns 1 a, 1 b,1 c, and 1 d as a carrier gas, and the first gas group can be introducedto the columns in sequence by switching, in sequence, the feed gas inletvalve in the downstream direction. Because gas of nitrogen, gas of CF₄and nitrogen, gas of CF₄, NF₃ and nitrogen, and gas of NF₃ and nitrogenflow out from each of the columns 1 a, 1 b, 1 c, and 1 d, in that order,the gases can be separated and discharged by switching a valve at theexit side in sequence and driving corresponding one of vacuum pumps 2 a,2 b, 2 c, and 2 d. The mixture fraction containing both CF₄ and NF₃ iscirculated to and joined with the feed first gas group. In this manner,gas of nitrogen, gas of CF₄ and nitrogen, and gas of NF₃ and nitrogenare obtained at the exit of the chromatographic separator.

[0018] It is preferable to perform collection of the gas for eachconstituent at the exit of the chromatographic separator and theswitching of the valves in a simulated-moving fed type chromatographicseparator 1 in FIG. 2 based on control conditions which are set based onthe gas composition of the first gas group or the performance of thefiller, or an analysis result of the gas at the exit. When the gascollection and valve switching are performed based on the analysisresult of the gas at the exit, gas constituents can be detected using,for example, a differential thermal detector (TCD) or Fouriertransform-infrared analyzer (FT-IR), and the control can be performedbased on the analysis. With this process, the gas is separated into itsconstituents, and thus, in the fractions for CF₄ and nitrogen, and forNF₃ and nitrogen, a pure mixture can be obtained with almost no othermaterials present.

[0019] The obtained fraction of CF₄ and nitrogen and the obtainedfraction of NF₃ and nitrogen are supplied to concentrators as necessary.As the concentrator, it is preferable to use a membrane separator or asubzero cooling device. In particular, by circulating the concentratedgas several times in a membrane separator or by using a multiple-stepmembrane separator, almost 100% of nitrogen can be separated, leaving apure, 100% concentration of CF₄ gas and NF₃ gas. The CF₄ gas and NF₃ gascan then be recovered and reused, for example, at the dry etchingprocess or cleaning process of a thin film forming device in thesemiconductor manufacturing process.

[0020] The above chromatographic separation by the chromatographicseparator and concentration by concentrator which is optional treatmentmeans are also similarly performed on the second gas group. In thismanner, pure, 100% concentration of C₂F₆ gas and CHF₃ gas can beobtained. Similar to the gases in the first gas group, C₂F₆ gas and CHF₃gas can be recovered and reused, for example, at the dry etching processor cleaning process of a thin film forming device in the semiconductormanufacturing process.

[0021] In the embodiment, by performing the upstream distillationseparation, the gas to be treated which contains, for example, fourspecific gases CF₄, NF₃, C₂F₆, and CHF₃, and another gas, nitrogen, canbe separated into three gas groups having different boiling points.Among the three gas groups, each of two gas groups is a mixture gas oftwo gas constituents. Thus, by further using chromatographic separators,CF₄ and NF₃, and C₂F₆ and CHF₃, which are each difficult to be separatedby other separation methods, can reliably be separated. The separatedgases of CF₄, NF₃, C₂F₆, and CHF₃ can then be recovered and reused.

[0022] As another embodiment, in a case where the gas to be treatedcontains three specific gases, CF₄, NF₃, and C₂F₆ or CHF₃, and anothergas, nitrogen, by performing the upstream distillation separation, thegas to be treated can be separated into three gas groups with differentboiling points, the first gas group including CF₄ and NF₃, the secondgroup including C₂F₆ or CHF₃, and the third group including nitrogen.Among the three gas groups, the mixture gas of CF₄ and NF₃ can beseparated into CF₄ and NF₃ using a chromatographic separator forseparating the specific gases. When the gas to be treated contains threespecific gases, C₂F₆, CHF₃, and CF₄ or NF₃, and another gas, nitrogen,by performing the upstream distillation separation, the gas to betreated can be separated into three gas groups with different boilingpoints, the first gas group including C₂F₆ and CHF₃, the second gasgroup including CF₄ or NF₃, and the third gas group including nitrogen.Among the three gas groups, the mixture gas of C₂F₆ and CHF₃ can beseparated into C₂F₆ and CHF₃ by further using a chromatographicseparator for separating the specific gases.

[0023] Other examples of PFC gases include C₃F₈ and SF₆, in addition toCF₄, NF₃, C₂F₆, and CHF₃. The former PFC gases can be separatedrelatively easily by various means. For example, C₃F₈ (having a boilingpoint of −36.7° C.) can be separated in the above example as a fourthgas group in the distillation separator. SF₆ (having a sublimationtemperature of −63.8° C. and melting point of −50.8° C.) obtained in thechromatographic separators 12 a and 12 b as a fraction that is furtherapart. Thus, fractions containing SF₆ can be separated from the fractionthat is separated as nitrogen in the above example.

[0024] Also, nitrogen is obtained at the distillation separator,chromatographic separator, and concentrators. Nitrogen, on the otherhand, is necessary as a diluting gas introduced before a vacuum pump inorder to dilute hydrogen fluoride in the discharge gas from thesemiconductor manufacturing process, and as the carrier gas for thechromatographic separator. It is therefore preferable to reuse theobtained nitrogen.

[0025] It is highly probable that the nitrogen gas to be reused containsvery small amounts of PFC gases. Therefore, it is preferable to treatthe nitrogen gas to remove the very small amounts of PFC gases withinthe nitrogen gas to be reused. As the process, it is preferable toemploy a known method for decomposing the PFC gases, such as, forexample, plasma decomposition process, combustion, and catalyst heatingprocess, or to repeat the distillation separation and chromatographicseparation, to separate the PFC gases and increase the purity ofnitrogen.

EXAMPLE 1

[0026] As a sample discharge gas, nitrogen gas was prepared whichcontained 1% each of CF₄, NF₃, C₂F₆, and CHF₃ (volume percent). TheSample discharge gas was introduced to a known distillation column wheredistillation operation was performed to separate the discharge gas intoa first gas group containing CF₄ and NF₃, second gas group containingC₂F₆ and CHF₃, and third gas group containing nitrogen. Theconcentration of CF₄ in the first gas group was 50%, the concentrationof NF₃ in the first gas group was 50%, the concentration of C₂F₆ in thesecond gas group was 50%, and the concentration of CHF₃ in the secondgas group was 50%.

[0027] The first gas group was passed through a chromatographicseparator which uses a column filled with silica gel and nitrogen as thecarrier gas. As a result, gas of CF₄ and gas of NF₃ were separated anddischarged in that order at the separator column exit, due to thedifference in the retention times. The purity of each gas was 100%.

[0028] Similarly, the second gas group was passed through achromatographic separator which uses a column filled with silica gel andnitrogen as the carrier gas. As a result, gas of C₂F₆ and gas of CHF₃were separated and discharged in that order at the separator columnexit, due to the difference in the retention times. The purity of eachgas was 100%.

[0029] An analysis of nitrogen obtained at the distillation separator 10and chromatographic separators 12 a and 12 b indicated that the nitrogengas contained 10 ppm of PFC gas. The PFC gas was decomposed to aconcentration of 0 ppm and neutralized by applying a plasmadecomposition process to the obtained nitrogen. It was thus confirmedthat the obtained nitrogen gas is usable as a diluting gas before thevacuum pump for diluting hydrogen fluoride within the discharge gas ofthe semiconductor manufacturing plant or as a carrier gas for thechromatographic separator.

[0030] According to the embodiment, by performing the upstreamdistillation separation, the gas to be treated which contains aplurality of specific gases and nitrogen as another gas can be separatedinto at least three gas groups with different boiling points, includingat least one group containing two gas constituents of which the boilingpoints are similar. Using the chromatographic separation for separatingspecific gas, the plurality of mixture gases can then be reliablyseparated into each constituent, the constituents being difficult to beseparated by other methods, such as, for example, CF₄ and NF₃, and C₂F₆and CHF₃. In other words, according to the present invention, aplurality of constituents, in particular, three or more PFC gasconstituents, that cannot be separated by a single separation methodsuch as distillation separation or chromatographic separation, can beseparated inexpensively and in high purity. The separated CF₄, NF₃,C₂F₆, and CHF₃ can be recovered and reused.

What is claimed is:
 1. A gas separation apparatus for separating atleast one specific gas from a gas to be treated which contains aplurality of specific gases, said apparatus comprising: a firstseparator for separating said gas to be treated into gas groups havingdifferent boiling points by distillation separation; and a secondseparator for separating specific gases by performing chromatographicseparation on at least one gas group separated by said first separator.2. A gas separation apparatus according to claim 1, wherein said secondseparator chromatographically separates a plurality of specific gaseshaving similar boiling points.
 3. A gas separation apparatus accordingto claim 1, wherein said gas to be treated contains PFC gases dischargedfrom a semiconductor manufacturing process as the specific gases andnitrogen as another gas.
 4. A gas separation apparatus according toclaim 3, wherein said PFC gases contain fluorine compounds having atleast one element of C, N, and S as the constituting element.
 5. A gasseparation apparatus according to claim 3, wherein said PFC gasesinclude at least CF₄ and NF₃.
 6. A gas separation apparatus according toclaim 5, wherein said CF₄ and NF₃ are separated into the same gas groupat said first separator and are separated from each other at said secondseparator.
 7. A gas separation apparatus according to claim 4, whereinsaid PFC gases include at least C₂F₆ and CHF₃.
 8. A gas separationapparatus according to claim 7, wherein said C₂F₆ and CHF₃ are separatedinto the same gas group at said first separator and are separated fromeach other at said second separator.
 9. A gas separation apparatusaccording to claim 1, wherein said second separator comprises aplurality of chromatographic columns; the column into which feed gasflows is sequentially switched among the plurality of chromatographiccolumns; and the function of each column is sequentially changed.
 10. Agas separation method for separating at least one specific gas from agas to be treated containing a plurality of specific gases, said methodcomprising the steps of: a first separation step for separating said gasto be treated into gas groups having different boiling points bydistillation separation; and a second separation step for separating thespecific gases by performing chromatographic separation on at least onegas groups that is distillation separated at said first separation step.11. A gas separation method according to claim 10, wherein in saidsecond separation step, a plurality of specific gases having similarboiling points are chromatographically separated.
 12. A gas separationmethod according to claim 10, wherein said gas to be treated containsPFC gases discharged from a semiconductor manufacturing process as thespecific gases and nitrogen as another gas.
 13. A gas separation methodaccording to claim 12, wherein said PFC gases include fluorine compoundshaving at least one element of C, N, and S as the constituting element.14. A gas separation method according to claim 13, wherein said PFCgases include at least CF₄ and NF₃.
 15. A gas separation methodaccording to claim 14, wherein said CF₄ and NF₃ are separated into thesame gas group at said first separation step and are separated from eachother at said second separation step.
 16. A gas separation methodaccording to claim 13, wherein said PFC gases include at least C₂F₆ andCHF₃.
 17. A gas separation method according to claim 16, wherein saidC₂F₆ and CHF₃ are separated into the same gas group at said firstseparation step and are separated from each other at said secondseparation step.